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chapter 3b - CH301 Chapter 3b Molecular Orbitals 1...

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1 CH301 Chapter 3b Molecular Orbitals 2 Molecular Orbital Theory z The diatomic molecule of O 2 has the following Lewis structure: •• •• O=O •• •• z The molecular geometric shape indicates that the molecule has a double bond and each oxygen atom has two unshared pairs of electrons around the central atom. 3 Molecular Orbital Theory z Since there are no half-filled orbitals (unpaired electrons): z It would be expected that the molecule, when placed in a magnetic field, would be diamagnetic. z Diamagnetic substances are those substances in which all the electrons are paired and are weakly repelled by magnetic fields. sp 3 sp 3 sp 3 sp 3
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4 Molecular Orbital Theory However, experimental procedures show that the molecule is paramagnetic. z Paramagnetic substances are those substances that contain unpaired electrons and are weakly attracted into magnetic fields. sp 3 sp 3 sp 3 sp 3 5 Figure 14.42: When liquid oxygen is poured into the space between the poles of a strong magnet, it remains there until it boils away. Source: Donald Clegg 6 Molecular Orbital Theory Therefore, the valence bond theory is not an acceptable explanation for the bonding that occurs in the oxygen molecule. z Molecular orbitals is a bonding theory that explains the phenomenon of the paramagnetic behavior of molecular oxygen by rationalization that O 2 has two unpaired electrons .
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7 Molecular Orbital Theory Combination of atomic orbitals on different atoms forms molecular orbitals (MO’s) so that electrons in MO’s belong to the molecule as a whole. z That is, we are going to account for all of the electrons in a molecule, not just the valence electrons. 8 Molecular Orbital Theory Electrons occupy orbitals called molecular orbitals . z Valence electrons are not localized on the atoms or between pairs of atoms… z The valence electrons are delocalized over the entire molecule 9 Molecular Orbital Theory Bonding orbitals z Linear combination of atomic orbitals. z LCAO – MO z The two atomic orbitals are treated as waves that are centered on different nuclei. z If the waves between nuclei have a constructive interference with each other z The total amplitude of the wavefunction is increased where the orbitals overlap
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10 Molecular Orbital Theory Waves that describe atomic orbitals have both positive and negative phases or amplitudes. As MO’s are formed the phases can interact constructively or destructively. 11 Molecular Orbitals z There are two simple types of molecular orbitals that can be produced by the overlap of atomic orbitals. 1. Head-on overlap of atomic orbitals produces σ (sigma) orbitals. 2. Side-on overlap of atomic orbitals produces π (pi) orbitals.
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